Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-05-07
2004-08-24
Hightower, P. Hampton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S420000, C525S425000, C525S437000, C525S451000, C524S538000, C524S539000, C524S602000
Reexamination Certificate
active
06780941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is generally aimed at a process for the preparation of melt spun, melt-colored, fibers. In particular, the invention relates to a process to form melt-colored fibers, from blends of at least one fiber-forming polyamide with at least one polyester, that exhibit improved color and aesthetics in comparison with equivalent melt-colored fibers manufactured using polyamide alone. In addition, the melt-colored fibers also exhibit improved dimensional stability when the fibers are exposed to changes in temperature and/or humidity.
2. Description of Related Art
Coloration of fibers has a long history, and the science of dyeing, initially of natural fibers such as flax, cotton and wool, has been under continuous development since Neolithic times. The appearance of man-made fibers, (e.g., cellulosics, acrylics, polyamides and polyesters), stimulated further developments in dyeing, and this method of coloring of fibers and articles made therefrom continues to be the most-practised technique for the production of colored fiber-based articles of manufacture.
In the case of fibers based on the more recent polymers such as polyamides and polyesters, which are spun from the melt, there exists an alternative method for coloration, i.e., addition of the colorant species into the melt and direct extrusion of colored fibers. While such a process may be carried out with dyes, it is more often carried out with pigments. Notwithstanding this fact, the process is popularly known throughout the industry as “solution dyeing”. The major difference between dyes and pigments is that, under prevailing processing conditions, pigments are virtually insoluble in polymers, whereas dyes are soluble, (see definitions in German Standards DIN 55943, 55944 and 55949, incorporated herein by reference).
As the technique of melt-pigmentation has been developed, it has been demonstrated that fibers made in this way can exhibit certain advantages over those made by post-spinning dyeing of fibers. Such advantages include improved resistance to degradation and fading in sunlight; lower susceptibility to fading and/or yellowing by polluting gases in the atmosphere, such as ozone and nitrogen oxides; improved resistance to chemicals, either in dry-cleaning processes or encountered in accidental spillages; less leaching or fading of color during laundering or cleaning processes involving water and detergents; no need for post-spinning industrial processes to color the products or to fix the color in place.
However, melt-pigmentation is also considered to have some disadvantages in terms of the color and appearance obtained in the final fiber. The fibers are generally regarded by those skilled in the art to exhibit degrees of lustre and low brightness that can render the said fibers unsuitable in certain applications.
The color change resulting from the addition of pigments to polymers is based on the wavelength-dependent absorption and scattering of light, with the appearance and color of the final product being a combination of these two factors as described in the Kubelka-Munk theory. A description of this theory, along with the general concepts of color and its measurement, may be found in “Colour Physics for Industry”, Roderick McDonald, (Ed.), The Society of Dyers and Colourists, Bradford, UK, 2
nd
Edition, (1997). Dyes can only absorb light and not scatter it, since the physical prerequisite for scattering—a certain minimum particle size—does not exist in the case of dyes in molecular solution; these colors are therefor transparent. Insofar as the transparency may be said to be attributable to the dye, complete absorption of light will result in black shades, selected absorption will result in colored shades.
The optical effect of pigments may in the same way be based on light absorption. If, however, the refractive index of the pigment differs appreciably from that of the polymer which is almost invariably the case, and if a specific particle size range is present, scattering takes place. Under these conditions, the initially transparent polymer becomes white and opaque, or, if selective absorption takes place at the same time, colored and opaque.
No scattering occurs when the particle sizes are very small, and none or very little occurs if they are very large. With all colored pigments that selectively absorb, the shade and strength of the final color is thus influenced by particle size. The transparency and thickness of the colored substrate may additionally affect the color strength. While some pigments are available in so-called transparent grades, e.g., red and yellow iron oxides, a complete color range across the spectrum is not readily available. Many such ultra-low particle size colorants are expensive, and difficult to maintain at high dispersion when compounded into a polymer matrix. It is also known that very low particle size additives in polymer melts can produce a profound effect on the Theological properties of said melt, resulting in formulations which are difficult to spin using standard equipment and procedures. Use of large particle size colorants is not a viable option either, as such additives will result in the blocking of spinneret orifices, pressure problems with filtration systems, and will lead to unacceptable levels of filament breaks in fiber production.
In any case, a large number of melt-pigmented fiber products are required to be opaque, and the problem lies in producing colored fibers with levels of color brightness close to those of dyed products. Note that a fundamental difference between dyeing and pigmenting of fibers is that, while dyes, or their mixtures, are either colored or absorb all wavelengths of light, (i.e., give black shades), pigmentation introduces an extra variable in that white pigments are readily available, whereas there is no such species as a “white dye”, nor can any combination of dyes result in a white fiber.
Another problem with particulate colorants in a polymeric melt-spun fiber is the phenomenon of dichroism, or optical anisotropy. Pigment particles are not necessarily isotropic in shape, and indeed may be needle-shaped, rod-shaped, or platelets. They may thus become oriented in a preferred direction due to the forces they encounter during processing of the melt and of the fiber. The apparent color then depends on the direction of observation. The origin of this phenomenon is to be found in the fact that certain pigments crystallise in crystal systems of low symmetry, resulting in directionally dependant physical properties. As far as the coloristic properties are concerned, this means that the absorption and scattering constants differ in the various principal crystallographic axes, i.e. such crystals are optically anisotropic.
With regard to the final fiber, as opposed to the above comments on the pigments themselves, the appearance of a sample thereof can vary depending on the angle of illumination and/or observation. Fiber samples are normally prepared for color and appearance testing by carefully wrapping the fiber or yarn sample, under conditions of uniform tension and consistent positioning of the said fibers, around a flat “card”, and assessing the color properties, and, more importantly, any differences between said properties and those of the desired sample or data, under standard conditions of illumination and observation. This may be carried out visually, but more usually is carried out using instrumentation. Methods and apparatus for carrying out the analysis of color and appearance in this manner are well known to those skilled in the art, and are not discussed in detail herein.
During such examinations, there are two additional effects that might be observed if the sample is illuminated or observed at a number of different angles. An example of multi-angle appearance testing of materials is reported in U.S. Pat. No. 4,479,718, assigned to DuPont. The first possible effect is that the total amount of light reflected from the sample, per unit area, may change. The second possib
McSheehy, Jr. Brendan Francis
Roth, Jr. Arthur
Studholme Matthew B.
Hampton Hightower P.
Kerins John C.
Miles & Stockbridge P.C.
Prisma Fibers Inc.
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